The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor; and, more particularly, to a CMOS image sensor with reduced interference and cross-talk phenomena occurring between closely located pixels by forming a differently structured photodiode in a pixel of a low power dissipation and high density CMOS image sensor and a method for fabricating the same.
Image sensor is a semiconductor device converting an optical image into an electric signal. Particularly, a charge coupled device (CCD) is a device, wherein each metal-oxide-silicon (hereinafter referred as to MOS) capacitor is closely located and carriers are stored into the MOS capacitor and transferred. A complementary metal oxide semiconductor (hereinafter referred as to CMOS) image sensor employs CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits and adopts a switching mode sensing outputs sequentially. The MOS transistors are formed as the same number of existing pixels in the peripheral circuit.
There are several problems in using the CCD due to its complex driving mode, high power dissipation, a complex process having lots of steps for a mask process and a difficulty in one chip realization since the signal processing circuit cannot be constructed on a CCD chip. Therefore, there has been actively researched on the CMOS image sensor that uses sub-micron CMOS technology to overcome the above problems. The CMOS image sensor obtains an image by forming a photodiode and a MOS transistor in a unit pixel and then detecting signals sequentially through a switching mode. The use of the CMOS technology results in less power dissipation and an enabled one chip process for the signal processing circuit. Also, compared to the CCD process that requires approximately 30 to 40 masks, the CMOS image sensor implemented with the CMOS technology needs approximately 20 masks because of a simplified process. Hence, the CMOS image sensor is currently highlighted as a next generation image sensor.
FIG. 1 is a cross-sectional view showing a photodiode formed in each unit pixel of a CMOS image sensor and a doping profile of ion implantation regions of the photodiode in accordance with a prior art.
Typically, a color image sensor has a plurality of arrayed pixels for red, green and blue colors. Hereinafter, a pixel for red is expressed as a red pixel and the same is applied for the other two colors. A photodiode of each pixel in accordance with the prior art has the identical structure for all red, green and blue pixels. Any one of three color filters (not shown) is formed on a top portion of this photodiode, and thus, each pixel is able to sense any one light among red, green and blue lights.
Referring to FIG. 1, among the red, green and blue lights, the blue light has the shortest penetration depth while the red light has the longest penetration depth. The red light is able to penetrate into neighboring pixels, and this ability further induces noises. The more detailed explanation on this effect will be provided in the following.
With reference to FIG. 1, a structure of a photodiode in accordance with a prior art will be described in detail. A field oxide layer 11 defining an active area and a field area is formed on a p-type substrate 10. Next, a p-type ion implantation region 12 is formed with a consistent depth from a surface of the p-type substrate 10.
Beneath the p-type ion implantation region 12, a first n-type ion implantation region 13 contacting to the p-type ion implantation region 12 is formed. Herein, the first n-type ion implantation region 13 has a high concentration and a consistent depth. A second n-type ion implantation region 14 contacting to the first n-type ion implantation region 13 with a consistent depth is formed beneath the first n-type ion implantation region 13. Herein, the second n-type ion implantation region 14 has a low concentration.
Generally, the field oxide layer 11 has a thickness ranging from about 0.3 xcexcm to about 0.8 xcexcm. Also, the second n-type ion implantation region 14 has a thickness in a range between about 0.3 xcexcm and about 0.8 xcexcm.
The p-type ion implantation region 12 formed on a near surface of the p-type substrate 10, the first n-type ion implantation region 13 formed below the p-type ion implantation region 12, the second n-type ion implantation region 14 and the p-type substrate 10 constitute a pn junction, constructing a p/n/p photodiode.
FIG. 1 provides another diagram showing a doping profile of the ion implantation regions measured in a logarithmic scale in accordance with the cross-sectional view of the photodiode illustrated in the left side of FIG. 1. This doping profile includes doping concentrations of the p-type ion implantation region P012, the first n-type ion implantation region N+ 13, the second n-type ion implantation region N 14 and the p-type substrate P-sub 10.
Also, this diagram shows a scale of a depletion region formed when a predetermined voltage is supplied to the pn junction having the above doping profile. It is indicated that the depletion region has a depth in several xcexc ms by being formed deeply into a deep region of the p-type substrate 10.
As well known, photodiode is a device that stores optical charges of light into the depletion region and uses the stored optical charges for generating an image as the photodiode supplies a predetermined voltage to the pn junction so that the depletion region is formed.
The photodiode constructed with a conventional structure has all identical depths for the red pixel, blue pixel and green pixel even though each color light has a different penetration depth. Therefore, since the depletion regions are formed even in deeper regions of the p-type substrate 10 of the blue pixel and the green pixel, red light penetrated into the neighboring red pixel induces light interference.
Furthermore, it is a current trend of increasing demands for developing a color image sensor that can be installed in a highly integrated and low power consuming mobile telecommunication terminal. However, this image sensor has a unit pixel of which size is decreased in about half of the conventional unit pixel. In case of applying the 0.18 xcexcm technology, the unit pixel size is below about 4.0 xcexcmxc3x974.0 xcexcm.
As the unit pixel size decreases, it is focused to solve such problems of a signal distortion with respect to the blue pixel and the green pixel caused by red light having a deep penetration depth and electric interference between neighboring pixels.
In case of using the 0.18 xcexcm technology instead of using the generally used 0.5 xcexcm or 0.35 xcexcm technology, it is much difficult to isolate devices. Also, there is another difficulty when using the 0.18 xcexcm technology as an allowable noise level decreases to about half of a conventional noise level.
Furthermore, in case of employing the 0.18 xcexcm technology, the photodiode area decreases below about 70%, and a driving voltage also decreases below about 75% compared to the area acquired when using the 0.35 xcexcm technology. Therefore, efficiency on optical charge generation is expected to be below 50% compared to the 0.35 xcexcn technology.
In order to compensate the efficiency on optical charge generation, it is essential to increase ion implantation energy and ion implantation concentration so to increase generations of an electron-hole pair. However, this increase in the ion implantation energy conversely decreases an insulating distance between pn junctions of nearly located photodiodes. Hence, there occur electric noises between nearly located pixels due to a weakened insulating characteristic. For this reason, it is much emphasized to compensate the insulating characteristic.
It is, therefore, an object of the present invention to provide a complementary metal-oxide semiconductor (CMOS) image sensor with decreased electric and optical noises and signal distortion between neighboring pixels and a method for fabricating the same.
In accordance with an aspect of the present invention, there is provided a complementary metal-oxide semiconductor (CMOS) image sensor, comprising: a first conductive type semiconductor substrate providing a first photodiode for sensing light having a first wavelength and a second photodiode for sensing light having a second wavelength, shorter than the first wave length, being closely located to the first photodiode, wherein each first and second photodiode includes: a first ion implantation region of a first conductive type formed in a semiconductor substrate of the first conductive type; a second ion implantation region of a second conductive type, contacting to a bottom surface of the first ion implantation region; a third ion implantation region of the second conductive type having a concentration lower than the second ion implantation region, contacting to a bottom surface of the second ion implantation region; and a forth ion implantation region of the second conductive type having a concentration lower than the second ion implantation region, contacting to a bottom surface of the third ion implantation region; and wherein the second photodiode includes a fifth ion implantation region of a first conductive type, surrounding lateral surfaces of the third and the fourth ion implantation regions and contacting to a bottom surface of the fourth ion implantation region and having a concentration higher than the third and the fourth ion implantation regions.
In accordance with another aspect of the present invention, there is also provided a complementary metal-oxide semiconductor (CMOS) image sensor, comprising: a first conductive type semiconductor substrate providing a first photodiode for sensing light having a first wavelength, a second photodiode for sensing light having a second wavelength, shorter than the first wave length, a third photodiode for sensing light having a third wavelength, shorter than the second wave length, being closely located to the first photodiode, wherein the first photodiode includes: a first ion implantation region of a first conductive type formed in a semiconductor substrate of the first conductive type; a second ion implantation region of a second conductive type, contacting to a bottom surface of the first ion implantation region; a third ion implantation region of the second conductive type having a concentration lower than the second ion implantation region, contacting-to a bottom surface of the second ion implantation region; and a forth ion implantation region of the second conductive type having a concentration lower than the second ion implantation region, contacting to a bottom surface of the third ion implantation region; and wherein the second photodiode includes: a fifth ion implantation region of a first conductive type formed in a semiconductor substrate of the first conductive type; a sixth ion implantation region of a second conductive type, contacting to a bottom surface of the fifth ion implantation region; a seventh ion implantation region of the second conductive type having a concentration lower than the sixth ion implantation region, contacting to a bottom surface of the sixth ion implantation region; a eighth ion implantation region of the second conductive type having a concentration lower than the sixth ion implantation region, contacting to a bottom surface of the seventh ion implantation region; and a ninth ion implantation region of a first conductive type, surrounding lateral surfaces of the seventh and the eighth ion implantation regions and contacting to a bottom surface of the eighth ion implantation region, and having a concentration higher than the seventh and the eighth ion implantation regions; and wherein the third photodiode includes: a tenth ion implantation region of a first conductive type formed in a semiconductor substrate of the first conductive type; a eleventh ion implantation region of a second conductive type, contacting to a bottom surface of the tenth ion implantation region; a twelfth ion implantation region of the second conductive type having a concentration lower than the eleventh ion implantation region, contacting to a bottom surface of the eleventh ion implantation region; a thirteenth ion implantation region of the first conductive type having a concentration lower than the twelfth ion implantation region, contacting to a bottom surface of the twelfth ion implantation region; and a fourteenth ion implantation region of a first conductive type, surrounding lateral surfaces of the twelfth and the thirteenth ion implantation regions and contacting to a bottom surface of the thirteenth ion implantation region, and having a concentration higher than the twelfth ion implantation region.
In accordance with further aspect of the present invention, there is also provided a method for forming a CMOS image sensor, wherein the CMOS image sensor including: a first photodiode for sensing light having a first wavelength; a second photodiode for sensing light having a second wavelength being closely located to the first photodiode, the method comprising the steps of: forming a field oxide layer on a substrate thereby defining an active area and; forming a fifth ion implantation region of a first conductive type in a second photodiode area provided in the substrate; expanding the fifth ion implantation region through a thermal process; forming first ion implantation regions of the first conductive type in areas of the first and the second photodiodes; forming forth ion implantation regions of a second conductive type in areas of the first and the second photodiodes, the forth ion implantation regions being deeper than the first ion implantation region but shallower than the fifth ion implantation region and having a concentration lower than the fifth ion implantation region; forming third ion implantation regions of the second conductive type in areas of the first and the second photodiodes, the third ion implantation regions being deeper than the first ion implantations region but shallower than the forth ion implantation region and having a concentration lower than the fifth ion implantation region; and forming second ion implantation region of the second conductive type in areas of the first and the second photodiodes, the second ion implantation regions formed in between the first ion implantation region and the third ion implantation region.
The present invention provides a preferred embodiment, wherein a photodiode structure of a red pixel is different from the photodiode structure of a blue and a green pixel. This differently structured photodiode decreases electric and optical noises occurring between neighboring pixels as simultaneously as improves light sensitivity.
In another preferred embodiment of the present invention, each of the red, blue and green pixels has different photodiode structure. Therefore, it is possible to optimize the photodiode structure for each light having a different color. As a result of this optimization, it is further possible to improve light sensitivity as simultaneous as to decrease signal interference between neighboring pixels.